Conventional transmission-type Liquid Crystal Displays (LCDs) exhibit high contrast ratios with good color saturation. However, their power consumption is high due to the need of a backlight. At bright ambient, e.g. outdoor, the display is washed out completely and hence loses its legibility. On the other hand, a reflective LCD uses ambient light for reading out the displayed images and hence retains its legibility under bright ambient. Their power consumption is reduced dramatically due to the lack of a backlight. However, the readability of a reflective LCD is lost under poor ambient light. In addition, its contrast ratio is also lower than that of the transmission-type LCD.
In order to overcome the above inadequacies, transflective LCDs (TLCD) have been developed to allow good legibility under any ambient light environment. In these displays the pixel is divided into R (reflective) and T (transmissive) sub-pixels. The T sub-pixel doesn't have a reflector so that it allows light from backlight to pass through and the device can operate in the transmission mode. Usually, the R and T area ratio is 4:1, in favor of the reflective display. The transmission mode is used for dark ambient only in order to conserve power. In general, there are two main approaches of transflective LCDs (TLCD) that have been developed: single cell gap (FIG. 1a) and double cell gap (FIG. 1b).
In the single cell gap approach, the cell gap (d) for R and T modes is the same. The cell gap is optimized for R-mode. As a result, the light transmittance for the T mode is generally 50% or lower because the light only passes the LC layer once. In order to achieve high light efficiency for both R and T modes, the double cell gap approach is often used such that the cell gap for the T pixels is twice as large as that for R pixels as shown in FIG. 1b. In this case the total length traveled by light in the LC layer is the same for both T and R. This approach however is suitable only for the ECB (Electrically Controlled Birefringence) modes, e.g. Vertical Alignment (VA) and Parallel Alignment (PA) modes.
Single cell gap transflective LCD (TLCD) usually leads to low efficiency for the transmission T. In order to attain high T and R, one often needs to turn to the double cell gap approach. This approach however leads to a much more complicated structure as well as a very demanding fabrication process. The fabrication process needs to have good control over the difference between the two cell gaps, which depends on the control of the extra layer (usually organic). This good control can be difficult which results in non-uniformity in the cell gap and hence deterioration of the LCD optical performance. Moreover, this difference in cell gap between R and T regions also leads to different response times between T and R displays modes.
These difficulties are best illustrated using a transflective LCD (TLCD) with a VA (Vertical alignment) LC mode. For example, if the cell gap(d) is the same for both R and T as shown in FIG. 2a, due to the double-path experienced by R, the reflected light R would have experienced a total retardation change of 2.Δn.d which is twice as large as that of T which is Δn.d. Hence the rate of reflection change is twice as fast as that of T, resulting in unequal light level change as shown in FIG. 2b. Here R reaches 100% brightness at 2.75V whereas T only reaches 50% at the same voltage. Thus a transflective LCD (TLCD) using this structure would have the on-state voltage, Von, at 2.75V which leads to only 50% light efficiency for T.
On the other hand, in the double cell gap approach as shown in FIG. 3a, the cell gap in the R region is reduced to d/2 so that the total path length for R (double-path) remains equal to d=(2×d/2) which is the same as that of T. This structure results in equal retardation change and brightness change for both R and T as shown in FIG. 3b. Both R and T thus can have high efficiency of 100%.
So far there have been very few approaches that can overcome the problems of the prior art teachings, i.e. to attain high light efficiencies using only a single cell gap. One possibility which was proposed by U.S. Pat. No. 6,281,952 is to use different LC alignments in the R and T regions. This approach is however very difficult to be achieved for mass production using the present LC technology.
A search in the United States Patent Office of the subject matter of this invention (hereafter disclosed) developed the following 7 U.S. patents and 2 published U.S. patent applications:
U.S. Pat. No. 4,256,377 to Krueger, et al is concerned with the development of an alignment for producing vertical alignment which has little to do with partial switching for TLCDs;
U.S. Pat. No. 5,113,273 to Mochizuki, et al is concerned with the improvement of the memory of an electro-optic response of ferroelectric liquid crystals;
U.S. Pat. No. 5,128,786 to Yanagisawa is about Black Matrix used for TFT-LCD devices which is of no relevance to the invention claimed herein;
U.S. Pat. No. 5,400,047 to Beesely is about the improvement of the response time of an electroluminescent display with no discussion of partial switching;
U.S. Pat. No. 5,515,189 to Kuratomi, et al is concerned with LC spatial light modulators for a neural network and not for transflective direct-view displays;
U.S. Pat. No. 6,043,605 to Park improves plasma displays by a floating auxiliary electrode which teaching is not relevant to LCDs;
U.S. Pat. No. 6,344,080 B1 to Kim, et al (as is the foregoing citation) is relevant only to plasma displays;
U.S. Pat. No. Publication 2001/0040666 A1 to Park although it teaches an alignment film for LCDs does not disclose any technique for generating TLCDs; and,
U.S. Pat. No. Publication 2001/0043297 A1 to Arai does not involve partial switching and is concerned with Twisted Nematic (TN) and Super Twisted Nematic LCDs.
None of the references developed in the search provided any suggestions for reducing the difficulties faced to attain high light efficiencies using only a single cell gap for its mass production using the present LC technology.